The present invention relates to a method for determining a dynamic response of a microstructure, and more particularly to a method for determining a dynamic response of a microstructure by using a pulsed broad bandwidth ultrasonic transducer as a bulk acoustic wave (BAW) hammer to excite the microstructure. The present invention also relates to an apparatus for determining a dynamic response of a microstructure.
Micro-sensors and micro-actuators are the key components in a micro electrical mechanical system (MEMS). The performance of a micro-sensor or micro-actuator correlates closely with the dynamic mechanical properties thereof. For instance, the bandwidth, resolution, and response time of some micro-sensors are determined by their mechanical resonance. The output characteristics of micro-actuators such as the force amplitude and the operating frequency thereof are also determined by their dynamic behaviors. Therefore, the testing method for evaluating the dynamic behaviors of the microstructures is very important. Several excitation and detection approaches have been developed to characterize the dynamic responses, vibration characteristics such as the natural frequencies and the mode shapes of the microstructures. Moreover, the material properties, e.g. residual stress, Young""s modulus and fatigue properties, can also be determined.
The measured dynamic response of a microstructure will be affected by the excitation technique. Please refer to FIG. 1 which schematically shows a conventional excitation device which drive the microstructure through built in electrostatic electrodes. The microstructure 10 formed on a silicon substrate 11 by a semiconductor manufacturing process is an insulator cantilever, for example, made of silicon oxide. In order to allow the cantilever 10 to be excited, a conductive film 12 such as a chromium film is applied over the insulator cantilever 10. Then, a variable-frequency sinusoidal voltage could be applied between the silicon substrate 11 and the metallized line 12 leading to the cantilever 10 by way of a variable frequency oscillator 13. Accordingly, the cantilever 10 with the chromium film 12 can be electrostatically attracted toward the substrate with either voltage polarity so as to excite the mechanical motion of the cantilever 10. In this approach, an additional conductive film which does not belong to the microstructure is deposited. Therefore, this test method is a destructive one. On the other hand, the presence of the additional film 12 may influence the dynamic behavior of the original cantilever 10.
FIG. 2 schematically shows another conventional excitation device which mechanically excites a microstructure. As shown, a test chip 20 with a microstructure (not shown) is attached onto a piezotransducer 21, and a voltage 22 is applied for driving the piezotransducer 21 so as to mechanically excite the test chip 20. The piezotransducer 21 is made of PZT. The natural frequencies of a PZT disc are strongly dependent on the ratio of diameter/thickness (D/T), and a PZT disc with finite dimension has complex mode distribution in the frequency domain. Accordingly, when a PZT transducer acts as the based excitation source applied to a microstructure, it is likely to be strongly coupled with the dynamic responses of the microstructure in the frequency range of a spurious mode of the PZT transducer. In brief, a dynamic coupling effect will exist between the piezotransducer 21 and the test chip 20 so as to interfere with the dynamic responses of the microstructure.
Further refer to FIG. 3 which shows a further conventional excitation device which uses a swept-sine signal to drive a microstructure. As shown, a specimen 31 with a microstructure (not shown) is attached onto a PZT transducer 30. By providing a dynamic signal analyzer 32, a swept-sine signal is generated to drive the PZT transducer 30 and further the specimen 31. As known, a swept-sine signal generated by a dynamic signal analyzer typically has frequencies under 50 kHz so as to be suitable for a millimeter dimensional microstructure. As for a micron dimensional microstructure with higher natural frequencies, higher exciting frequencies will be required.
FIG. 4 shows a still further conventional excitation device which uses acoustic waves to excite a microstructure. As shown, a small loudspeaker 41 is mounted above a cantilever 40 to be excited. By providing a power for the loudspeaker 41, the acoustic waves 43 propagate via the air to the cantilever 40, thereby forcing the cantilever 40 to vibrate. In this approach, the acoustic waves 43 have to be transmitted to the cantilever 40 via air so that the cantilever 40 cannot be excited in a vacuum environment where micron dimensional microstructures are possibly located.
Therefore, an object of the present invention is to provide a method and/or apparatus for determining a dynamic response of a microstructure, in which no additional film is deposited on the microstructure to be excited.
Another object of the present invention is to provide a method and/or apparatus for determining a dynamic response of a microstructure, in which the dynamic coupling effect between the transducer and the microstructure is minimized.
A further object of the present invention is to provide a method and/or apparatus for determining a dynamic response of a microstructure, which can be used for a micron dimensional microstructure.
A still further object of the present invention is to provide a method and/or apparatus for determining a dynamic response of a microstructure, which can be operated in a vacuum environment.
According to a first aspect of the present invention, a method for determining a dynamic response of a microstructure includes steps of attaching the microstructure to an ultrasonic transducer device; providing a pulse voltage to excite the ultrasonic transducer device so as to generate a pulsed bulk acoustic wave which has a bandwidth of at least 20%; and utilizing free vibration of the microstructure resulting from the pulsed bulk acoustic wave to determine the dynamic response of the microstructure. Preferably, the bandwidth is no less than 30%.
The microstructure is preferably attached to the ultrasonic transducer device by adhering a substrate of the microstructure to the ultrasonic transducer device in a nondestructive manner. For example, the substrate is adhered to the ultrasonic transducer device by wax or a sticky tape.
Preferably, the ultrasonic transducer device is a piezocomposite ultrasonic transducer. More preferably, the ultrasonic transducer device includes a piezoelectric portion and a polymer portion around the piezoelectric portion.
A second aspect of the present invention relates to a method for determining a dynamic response of a microstructure includes steps of attaching the microstructure to a piezocomposite ultrasonic transducer device formed of a piezoelectric material and a polymer material around said piezoelectric ceramic material; providing a pulse voltage for the piezocomposite layer to excite the ultrasonic transducer device so as to generate a pulsed bulk acoustic wave; and utilizing free vibration of the microstructure resulting from the pulsed bulk acoustic wave to determine the dynamic response of the microstructure.
Preferably, the piezoelectric material is PZT, and the polymer is epoxy resin or silicone.
Preferably, the pulsed bulk acoustic wave has a bandwidth of at least 20%, and more preferably, at least 30%, and a central frequency of hundreds of kHz to tens of MHz.
A third aspect of the present invention relates to an apparatus for determining a dynamic response of a microstructure includes a pulse generator for providing a pulse voltage; a piezocomposite ultrasonic transducer device including a plurality of piezoelectric ceramic rods filled with a polymer therebetween, and connected to said pulse generator for generating a pulsed bulk acoustic wave in response to said pulse voltage to vibrate said microstructure secured thereon; a detecting device positioned to detect the vibrating microstructure for determining the dynamic response of the microstructure.
Preferably, the piezocomposite ultrasonic transducer is divided into three layers, i.e. piezocomposite layers, matching layers, and a backing layer. The matching layer is formed of epoxy resin for acoustic impedance matching; the piezocomposite layer consists of a PZT rod matrix filled with epoxy resin for emitting the pulsed bulk acoustic wave; and the backing layer is formed of epoxy resin or silicone for damping the acoustic wave from the rear surface of piezocomposit layer. If necessary, matching layers and/or piezocomposite layers can be repetitively provided. Alternatively, matching layers and/or the backing layer can be omitted. Further, piezoelectric layers such as PZT layers can also be used along with the matching and backing layers to achieve the purpose of the present invention.
Preferably, the detecting device includes a laser Doppler vibrometer positioned above the microstructure for monitoring the dynamic response of the vibrating microstructure as a photo-signal; a photoelectric converter electrically connected to the laser Doppler vibrometer for converting the photo-signal into an electric signal; and an oscilloscope electrically connected to the photoelectric converter for displaying the dynamic response in response to the electric signal.
In an embodiment, the photoelectric converter is a charge coupled device (CCD).
The oscilloscope and the pulse generator can be separate devices interconnected to each other, or the oscilloscope can include therein the pulse generator.
For a microstructure used in a vacuum environment, a vacuum chamber is further provided for accommodating therein the ultrasonic transducer device and the microstructure to perform the determination of the dynamic response.
The method and/or apparatus according to the present invention are suitable for determining a dynamic response of a torsional micro-mirror or a micro-cantilever or any other similar microstructure.